Three-time Tour de France champion Alberto Contador of Spain tested positive for a banned steroid while winning this year's race and has been suspended by cycling's governing body.

A World Anti-Doping Agency-accredited lab in Cologne, Germany, found a "very small concentration" of clenbuterol in Contador's urine sample on July 21 at the Tour, according to a statement from the UCI.

But the amount was "400 time(s) less than what the antidoping laboratories accredited by WADA must be able to detect," the cycling organization said.

Both Contador's A and B samples tested positive and the cyclist has been "formally and provisionally suspended," the UCI said.

If Contador is proved to have cheated, cycling would suffer a devastating blow. Having invested millions of dollars in recent years in what is widely regarded as the one of the most stringent anti-doping regimes anywhere, cycling authorities hopes to be turning the corner on widespread doping by riders that have long made a mockery of the sport. Although just 27, Contador is already the greatest rider of his generation. His victories at the Tour and elsewhere were seen as a possible break from cycling's dirty past.

The UCI's statement gave no indication of whether Contador will be stripped of his latest Tour title or be banned.

"The UCI continues working with the scientific support of WADA to analyze all the elements that are relevant to the case. This further investigation may take some more time," the statement said.

Jacinto Vidarte, Contador's publicist, released a statement saying the cyclist insists food contamination is the only possible explanation.

"The experts consulted so far have agreed also that this is a food contamination case, especially considering the number of tests passed by Alberto Contador during the Tour de France," Vidarte said in the release, "making it possible to define precisely both the time the emergence of the substance as the tiny amount detected, ruling out any other source or intentionality."

Contador will hold a news conference Thursday in Pinto, Spain.

WADA director general David Howman told The Associated Press on Thursday that testing positive for even the most minute amounts of clenbuterol could be enough to sanction an athlete, although he refused to discuss the specifics of Contador's case.

"The issue is the lab has detected this. They have the responsibility for pursuing. There is no such thing as a limit where you don't have to prosecute cases. This is not a substance that has a threshold," said Howman, reached by telephone as he was changing planes in Dubai on his way to the Commonwealth Games in India.

"Once the lab records an adverse finding, it's an adverse finding and it has to be followed up."

"Clenbuterol is a substance that has been used for over 20 to 30 years," he added. "It is not anything new. Nobody has ever suggested it is something you can take inadvertently."

Contador was first made aware of the positive test on Aug. 24, according to Vidarte's statement.

In July, Contador won the Tour de France for the third time in four years, beating Andy Schleck of Luxembourg by 39 seconds.

Contador is a lithe but powerful rider who likes to use explosive bursts of speed up sharp mountain climbs to shake off rivals. Having won all three Grand Tours of France, Italy and Spain, something seven-time Tour de France champion Lance Armstrong never achieved, he appeared destined to become one of cycling's all-time greats.

If Tour officials strip Contador of his title, he would be just the second cyclist so punished. The first was American Floyd Landis, who was stripped of his 2006 Tour title after a positive test. For years, Landis denied doping but admitted this spring that he used performance-enhancing drugs. In doing so, he accused Armstrong and others of systematic drug use.

Clenbuterol is used as a weight-loss drug which builds muscle while burning fat. It is commonly given to horses to treat breathing problems.

American swimmer Jessica Hardy tested positive for clenbuterol at the U.S. trials in July 2008.